This graph shows the amount of sunlight entering the top of
Earth’s atmosphere from 1610 to 2010. Scientists call this quantity
“total solar irradiance,” given here in watts per square meter (W/m2).
Space-based measurements, begun in 1978, indicate Earth receives an
average of 1,361 W/m2 of incoming sunlight, an amount that has varied in
the recent past by about 1 W/m2 (or one-tenth of one percent) on roughly
11-year cycles. Fortunately, incoming sunlight is fairly stable, and
while it has increased slightly over the last century, these changes
account for only a small fraction of the warming our world experienced
in that span. Scientists rely upon sunspot records to estimate solar
irradiance values before 1978.
(Data courtesy of Greg Kopp, Laboratory for Atmospheric and Space
Physics, University of Colorado; and Judith Lean, Space Science
Division, Naval Research Laboratory.
Graph courtesy of NOAA's ClimateWatch Magazine)